The purpose of this study is to examine the host immune response to viruses. The major goal is to examine the normal immune response to naturally occurring viruses in man and to extend these studies to patients in order to identify abnormalities of immune regulation which may be related to the pathogenesis of certain diseases of the nervous system. These studies involve a functional analysis of the cellular immune response to measles virus and other viruses of man. This includes studies of cytotoxic, helper and suppressor T-cell populations. The genetic influence on the generation and expression of these responses is being examined. In addition, T-cell lines and clones are also being used to examine cellular reactivity to these viruses.